Ultrasonic signal processor for a hand held ultrasonic diagnostic instrument

ABSTRACT

A hand held ultrasonic instrument is provided in a portable unit which performs both B mode and Doppler imaging. The instrument includes a transducer array mounted in a hand-held enclosure, with an integrated circuit transceiver connected to the elements of the array for the reception of echo signals. A digital signal processing circuit performs both B mode and Doppler signal processing such as filtering, detection and Doppler estimation, as well as advanced functions such as assembly of multiple zone focused scanlines, synthetic aperture formation, depth dependent filtering, speckle reduction, flash suppression, and frame averaging.

This application is a continuation-in-part of and claims the benefit ofpriority from application Ser. No. 08/672,782 filed Jun. 28, 1996, nowU.S. Pat. No. 5,722,412; application Ser. No. 08/826,543 filed Apr. 3,1997, now U.S. Pat. No. 5,893,363; and application Ser. No. 08/863,937filed May 27, 1997, now U.S. Pat. No. 5,817,024, the full disclosures ofwhich are incorporated herein by reference.

This invention relates to medical ultrasonic diagnostic systems and, inparticular, to a fully integrated hand held ultrasonic diagnosticinstrument.

As is well known, modern ultrasonic diagnostic systems are large,complex instruments. Today's premium ultrasound systems, while mountedin carts for portability, continue to weigh several hundred pounds. Inthe past, ultrasound systems such as the ADR 4000 ultrasound systemproduced by Advanced Technology Laboratories, Inc., assignee of thepresent invention, were smaller, desktop units about the size of apersonal computer. However, such instruments lacked many of the advancedfeatures of today's premium ultrasound systems such as color Dopplerimaging and three dimensional display capabilities. As ultrasoundsystems have become more sophisticated they have also become bulkier.

However, with the ever increasing density of digital electronics, it isnow possible to foresee a time when ultrasound systems will be able tobe miniaturized to a size even smaller than their much earlierancestors. The physician is accustomed to working with a hand heldultrasonic scanhead which is about the size of an electric razor. Itwould be desirable, consistent with the familiar scanhead, to be able tocompact the entire ultrasound system into a scanhead-sized unit. Itwould be further desirable for such an ultrasound instrument to retainas many of the features of today's sophisticated ultrasound systems aspossible, such as speckle reduction, color Doppler and three dimensionalimaging capabilities.

In accordance with the principles of the present invention, a diagnosticultrasound instrument is provided which exhibits many of the features ofa premium ultrasound system in a hand held unit. These premium systemfeatures are afforded by a digital signal processor capable ofperforming both greyscale and Doppler signal processing including theirassociated filtering, compression, flash suppression and mappingfunctions, as well as advanced features such as synthetic apertureformation, multiple focal zone imaging, frame averaging, depth dependentfiltering, and speckle reduction. In a preferred embodiments the digitalsignal processor is formed on a single integrated circuit chip. Thissophisticated ultrasound instrument can be manufactured as a hand heldunit weighing less than five pounds.

In the drawings:

FIG. 1 illustrates in block diagram form the architecture of a hand-heldultrasound system of the present invention;

FIGS. 2a and 2b are front and side views of a hand-held ultrasoundsystem of the present invention which is packaged as a single unit;

FIGS. 3a and 3b are front and side views of the transducer unit of atwo-unit hand-held ultrasound system of the present invention;

FIG. 4 illustrates the two units of a hand-held ultrasound system of thepresent invention in a two-unit package;

FIG. 5 is a block diagram of the digital signal processing ASIC of theultrasound system of FIG. 1;

FIG. 6 is a flowchart of B mode processing by the digital signalprocessing ASIC;

FIG. 7 is a flowchart of Doppler processing by the digital signalprocessing ASIC; and

FIG. 8 is a chart of the user controls of the ultrasound system of FIG.1.

Referring first to FIG. 1, the architecture of a hand-held ultrasoundsystem of the present invention is shown. It is possible to package anentire ultrasound system in a single hand-held unit only throughjudicious selection of functions and features and efficient use ofintegrated circuit and ultrasound technology. A transducer array 10 isused for its solid state, electronic control capabilities, variableaperture, image performance and reliability. Either a flat or curvedlinear array can be used. In a preferred embodiment the array is acurved array, which affords a broad sector scanning field. While thepreferred embodiment provides sufficient delay capability to both steerand focus a flat array such as a phased array, the geometric curvatureof the curved array reduces the steering delay requirements on thebeamformer. The elements of the array are connected to atransmit/receive ASIC 20 which drives the transducer elements andreceives echoes received by the elements. The transmit/receive ASIC 20also controls the active transmit and receive apertures of the array 10and the gain of the received echo signals. The transmit/receive ASIC ispreferably located within inches of the transducer elements, preferablyin the same enclosure, and just behind the transducer. A preferredembodiment of the transmit/receive ASIC is described in detail in U.S.patent application Ser. No. 08/826,543 filed Apr. 3, 1997, now U.S. Pat.No. 5,893,363 and entitled ULTRASONIC ARRAY TRANSDUCER TRANSCEIVER FOR AHAND HELD ULTRASONIC DIAGNOSTIC INSTRUMENT.

Echoes received by the transmit/receive ASIC 20 are provided to theadjacent front end ASIC 30, which beamforms the echoes from theindividual transducer elements into coherent scanline signals. The frontend ASIC 30 also controls the transmit waveform timing, aperture andfocusing of the ultrasound beam through control signals provided for thetransmit/receive ASIC. In the illustrated embodiment the front end ASIC30 provides timing signals for the other ASICs and time gain control. Apower and battery management subsystem 80 monitors and controls thepower applied to the transducer array, thereby controlling the acousticenergy which is applied to the patient and minimizing power consumptionof the unit. A memory device 32 is connected to the front end ASIC 30,which stores data used by the beamformer. A preferred embodiment of thefront end ASIC is described in detail in U.S. patent application Ser.No. 08/863,937 filed May 27, 1997, now U.S. Pat. No. 5,817,024 filed andentitled HAND HELD ULTRASONIC DIAGNOSTIC INSTRUMENT WITH DIGITALBEAMFORMER.

Beamformed scanline signals are coupled from the front end ASIC 30 tothe digital signal processing ASIC 40. The digital signal processingASIC 40 filters the scanline signals, processes them as B mode signals,Doppler signals, or both, and in the preferred embodiment also providesseveral advanced features including synthetic aperture formation,frequency compounding, Doppler processing such as power Doppler (colorpower angio) processing, and speckle reduction as more fully detailedbelow.

The ultrasound B mode and Doppler information is then coupled to theadjacent back end ASIC 50 for scan conversion and the production ofvideo output signals. A memory device 42 is coupled to the back end ASIC50 to provide storage used in three dimensional power Doppler (3D CPA)imaging. The back end ASIC also adds alphanumeric information to thedisplay such as the time, date, and patient identification. A graphicsprocessor overlays the ultrasound image with information such as depthand focus markers and cursors. Frames of ultrasonic images are stored ina video memory 54 coupled to the back end ASIC 50, enabling them to berecalled and replayed in a live Cineloop® realtime sequence. Videoinformation is available at a video output in several formats, includingNTSC and PAL television formats and RGB drive signals for an LCD display60 or a video monitor.

The back end ASIC 50 also includes the central processor for theultrasound system, a RISC (reduced instruction set controller) processor502. The RISC processor is coupled to the front end and digital signalprocessing ASICs to control and synchronize the processing and controlfunctions throughout the hand-held unit. A program memory 52 is coupledto the back end ASIC 50 to store program data which is used by the RISCprocessor to operate and control the unit. The back end ASIC 50 is alsocoupled to a data port configured as an infrared transmitter or a PCMCIAinterface 56. This interface allows other modules and functions to beattached to or communicate with the hand-held ultrasound unit. Theinterface 56 can connect to a modem or communications link to transmitand receive ultrasound information from remote locations. The interfacecan accept other data storage devices to add new functionality to theunit, such as an ultrasound information analysis package.

The RISC processor is also coupled to the user controls 70 of the unitto accept user inputs to direct and control the operations of thehand-held ultrasound system.

Power for the hand-held ultrasound system in a preferred embodiment isprovided by a rechargeable battery. Battery power is conserved andapplied to the components of the unit from the power subsystem 80. Thepower subsystem 80 includes a DC converter to convert the low batteryvoltage to a higher voltage which is applied to the transmit/receiveASIC 20 to drive the elements of the transducer array 10.

FIGS. 2a and 2b illustrate a one piece unit 87 for housing theultrasound system of FIG. 1. The front of the unit is shown in FIG. 2a,including an upper section 83 which includes the LCD display 60. Thelower section 81 includes the user controls as indicated at 86. The usercontrols enable the user to turn the unit on and off, select operatingcharacteristics such as the mode (B mode or Doppler), color Dopplersector or frame rate, and special functions such as three dimensionaldisplay. The user controls also enable entry of time, date, and patientdata. A four way control, shown as a cross, operates as a joystick tomaneuver cursors on the screen or select functions from a user menu.Alternatively a mouse ball or track pad can be used to provide cursorand other controls in multiple directions. Several buttons and switchesof the controls are dedicated for specific functions such as freezing animage and storing and replaying an image sequence from the Cineloopmemory.

At the bottom of the unit 87 is the aperture 84 of the curved transducerarray 10. In use, the transducer aperture is held against the patient toscan the patient and the ultrasound image is displayed on the LCDdisplay 60.

FIG. 2b is a side view of the unit 87, showing the depth of the unit.The unit is approximately 20.3 cm high, 11.4 cm wide, and 4.5 cm deep.This unit contains all of the elements of a fully operational ultrasoundsystem with a curved array transducer probe, in a single packageweighing less than five pounds. A major portion of this weight isattributable to the battery housed inside the unit.

FIGS. 3 and 4 illustrate a second packaging configuration in which theultrasound system is housed in two separate sections. A lower section 81includes the transducer array, the electronics through to a video signaloutput, and the user controls. This lower section is shown in FIG. 3a,with the curved transducer array aperture visible at the bottom. Thelower section is shown in the side view of FIG. 3b. This lower sectionmeasures about 11.4 cm high by 9.8 cm wide by 2.5 cm deep. This unit hasapproximately the same weight as a conventional ultrasound scanhead.This lower section is connected to an upper section 83 as shown in FIG.4 by a cable 90. The upper section 83 includes an LCD display 82 and abattery pack 88. The cable 90 couples video signals from the lower unit81 to the upper unit for display, and provides power for the lower unitfrom the battery pack 88. This two part unit is advantageous because theuser can maneuver the lower unit and the transducer 84 over the patientin the manner of a conventional scanhead, while holding the upper unitin a convenient stationary position for viewing. By locating the batterypack in the upper unit, the lower unit is lightened and easilymaneuverable over the body of the patient.

Other system packaging configurations will be readily apparent. Forinstance, the front end ASIC 30, the digital signal processing ASIC 40,and the back end ASIC 50 could be located in a common enclosure, withthe beamformer of the front end ASIC connectable to different arraytransducers. This would enable different transducers to be used with thedigital beamformer, digital filter, and image processor for differentdiagnostic imaging procedures. A display could be located in the sameenclosure as the three ASICS, or the output of the back end ASIC couldbe connected to a separate display device. Alternatively, the transducerarray 10, transmit/receive ASIC 20 and front end ASIC 30 could be in thetransducer enclosure and the balance of the system in the battery anddisplay unit. The configuration of FIG. 4 could be changed to relocatethe user controls onto the display and battery pack unit, with theultrasound ASICs located in the unit with the transducer array.

Referring to FIG. 5, a detailed block diagram of the digital signalprocessing ASIC 40 is shown.

Scanline signals from the front end ASIC 30 are received by anormalization circuit 410, where they are multiplied by a variablecoefficient supplied by coefficient memory 408 to normalize the receivedsignals for aperture variation. When the transducer is receiving signalsalong the scanline from shallow depths, a relatively small aperture,such as four or eight transducer elements, is used to receive echosignals. As the reception depth along the scanline increases, theaperture is incrementally increased so that the full 32 element apertureis used at maximum depths. The normalization circuit 410 will multiplythe received scanline signals by appropriate coefficients over the rangeof aperture variation, such as factors of four or eight, to normalizethe signals for this aperture variation effect.

When the ultrasound system is operated in the B mode to form astructural image of tissue and organs, the digital signal processor isoperated as shown by the flowchart of FIG. 6. The normalized echosignals follow two paths in FIG. 5, one of which is coupled to a fourmultiplier filter 412 and the other of which is coupled by a multiplexer422 to a second four multiplier filter 414. Each multiplier filterincludes a multiplier and an accumulator which operate as an FIR (finiteimpulse response) filter. Scanline echo signals are shifted sequentiallyinto a multiplier, multiplied by coefficients supplied by thecoefficient memory 408, and the products are accumulated in theaccumulator at the output of the multiplier. The coefficients for thefilter 412 are chosen to multiply the echo signals by a cosine functionand the coefficients for the filter 414 are chosen to multiply the echosignals by a sine function, preparatory for I and Q quadrature signaldetection. The four multiplier filters produce accumulated signals at arate which is less than the input rate to the multipliers, therebyperforming decimation band pass filtering. When the signal bandwidthexceeds the display bandwidth of the display monitor, the image lineswill flicker due to an aliasing condition. The decimation filtering isdesigned to reduce the signal bandwidth as well as the data rate tomatch the display bandwidth of the monitor. By applying a succession ofinput signals and coefficients to a multiplier and accumulatingintermediate products, the effective length of the filter can beincreased. For instance, input signals 1-8 can be sequentially weightedby the fourth multiplier and the products accumulated in the fourthaccumulator; input signals 3-10 can be weighted by the third multiplierand the products accumulated in the third accumulator; input signals5-12 can be weighted by the second multiplier and the productsaccumulated in the second accumulator; and input signals 7-14 can beweighted by the first multiplier and the products accumulated in thefirst accumulator. The data rate has thereby been decimated by two, andeach multiplier and accumulator is effectively operated as an eight tapfilter. Thus it is seen that the effective number of taps of the filteris a product of the number of multipliers (four in this example) and thedecimation rate (two in this example).

Additionally, this filter reduces r.f. noise and quantization noisethrough its bandwidth limiting effects. I and Q echo signal samples areproduced at the outputs of filters 412 and 414, amplified if desired bythe multipliers of gain stages 416 and 418, then stored in the r.f.memory 420. The Q samples are coupled to the r.f. memory by amultiplexer 426.

When a synthetic aperture image is to be formed, partially summedscanlines from a portion of the full aperture are acquired followingseparate pulse transmissions, then combined to form full aperturescanlines. When the synthetic aperture is formed from two pulsetransmissions, the I and Q samples from the scanline of the first halfof the aperture are stored in the r.f. memory 420 until the I and Qsamples from the other half of the aperture are received. As the samplesfrom the second half of the aperture are received, they are combinedwith their spatially corresponding counterparts by an adder 424. Thesize of the r.f. memory is kept to a minimum by storing the aperturesignals after decimation filtering, which reduces the size of the memoryrequired to store the scanline signal samples.

After the I and Q samples for the full aperture have been formed, theecho samples are coupled from the adder 424 to a detection andcompression circuit 428. This circuit includes two shift registers and amultiplier arranged to form a CORDIC processor for performing envelopedetection of the form (I² +Q²)^(1/2). See, for instance, "The CORDICTrigonometric Computing Technique," by J. E. Volder, IRE Trans. onElect. Computers, (September 1959). The detected signal is compressedand scaled to map the detected signals to a desired range of displaygray levels.

Following detection and compression mapping, the grayscale signals arelowpass filtered in an FIR filter 432, then stored in an image framememory 430. If the selected scanning mode utilizes a single transmitfocal point, the grayscale signals are transmitted to the back end ASIC50 for scan conversion. Prior to leaving the ASIC 40, the greyscalesignals can be frame averaged by an infinite impulse response (IIR)filter 436 which utilizes image frame memory 430 as a frame buffer andincorporates one multiplier and two adders to perform frame to frameaveraging of the form

F_(out) =(1-α) F_(out-1) +αF_(new) =F_(out-1) +α(F_(new) -F_(out-1))where the multiplier coefficient is α. If the coefficient is a binarynumber (e.g., 0.5, 0.25, 0.125) F_(out) can be obtained with anadd-shift-add operation.

If multiple focal zones are used, each received scanline segment isstored in the r.f. memory 420 until scanline segments from the entiredisplay depth have been received. Preferably the scanline segments forone complete focal zone are acquired before transmitting and receivingsegments from another focal zone. When all segments for a scanline havebeen acquired, each complete scanline is then read out of the r.f.memory and filtered by the FIR filter 432, which smoothes the boundariesbetween the segments for a more pleasing, artifact-free image.

If both multiple zone focusing and synthetic aperture are used, thescanline segments of both halves of the aperture are received over thefull focal zone and assembled in the r.f memory 420. Correspondingscanline segments are then received from other focal zones and joinedwith the segments from the first received focal zone. The completedscanlines are then filtered by FIR filter 432 to smooth the boundariesbetween segments.

The user may choose to process the grayscale image with certain imageenhancement features, such as depth dependent filtering or specklereduction such as the frequency compounding technique described in U.S.Pat. No. 4,561,019. These optional processing techniques necessitate theuse of the filters 412 and 414 for separate bandpass filtering of thescanline signals and absolute value detection rather than quadraturedetection. In the case of depth dependent filtering the received echosignals are multiplied by cosine functions in both of filters 412 and414, but with coefficients chosen so that one filter produces outputsignals in a high passband and the other produces output signals in alow passband. The output signals produced by the two filters are of theform I₁ =h₁ (t)cosω_(H) t and I₂ =h₂ (t)cosω_(L) t. These two outputsignals are amplified in gain stages 416 and 418 by complementary timevarying gain control functions. The high frequency passband signals I₁are initially amplified strongly, then the gain is decreased as echosignals are received from increasing depths along the scanline. In acomplementary manner the low frequency passband signals I₂ are initiallyat a low level, then amplified in an increasing manner with depth as thehigh frequency gain is rolled off. Thus, signals at shallow depths willexhibit a relatively high passband, and signals from greater depths willpass through a relatively lower passband which reduces high frequencynoise at the greater depths. Detection in the CORDIC processor ofcircuit 428 is performed by absolute value detection by squaring I₁ andI₂, then summing the results. Following summation the signals are logcompressed to the desired grayscale mapping characteristic.Alternatively, the signals passed by the separate passbands are summedby the adder 424, then detected by absolute value detection in thedetection and compression circuitry 428 and mapped.

The same processors can be used to provide speckle reduction byfrequency compounding. The coefficients of one of the filters 412,414are chosen to filter the received signals by a high frequency passband,and the coefficients of the other filter are chosen to filter thereceived signals by a contiguous low frequency passband. Thecoefficients of the gain stages 416,418 are chosen to equalize theresponses of the two passbands. The signals of the high and lowpassbands are coupled to the detection and compression circuitry wherethe passbands are separately detected through absolute value detectionas described above, then the detected signals are log compressed to thedesired grayscale mapping characteristic and summed on a spatial basis.

The processing of Doppler echo signals for power Doppler (CPA) displayis shown in FIG. 5 together with the flowchart of FIG. 7. Each scanlinevector is scanned repetitively, for instance eight times, to assemble anensemble of Doppler information along the vector. Each received scanlineof echo signals is normalized by the normalization circuit 410 andundergoes decimation band pass filtering in the filter 412. Eachscanline of the ensemble is stored in the r.f. memory 420 until acomplete ensemble has been accumulated. The scanlines of each ensembleare coupled by the multiplexer 422 to the four multiplier filter 414,which performs wall filtering and Doppler power estimation throughmatrix filtering. Wall filtering is performed by selection ofappropriate multiplier coefficients and the matrix filtering is of theform ##EQU1## where x₁ . . . x_(n) are spatially aligned signals fromthe ensemble of scanlines and y₁ . . . y_(n) are output Doppler values.In a preferred embodiment a four multiplier filter is used for matrixfiltering, and the filtering is performed sequentially andincrementally. Intermediate products are accumulated as described above,thereby extending the filter length. For example, in processing theabove matrix with a four multiplier filter, the intermediate productsa₁₁ x₁ +a₁₂ x₂ +a₁₃ x₃ +a₁₄ x₄ are formed initially and summed in theaccumulator. Then products a₁₅ x₅ +a₁₆ x₆ +a₁₇ x₇ +a₁₈ x₈ are formed bythe multipliers and summed in the accumulator with the previouslycomputed intermediate products. By accumulating intermediate products inthis manner the four multipliers and accumulator can be extended to afilter of any desired length, restricted only by the maximum processingtime available. The Doppler values are coupled to the detection andcompression circuitry 428 through the gain stage 418 and the multiplexer426, where the Doppler signal amplitude at each echo location along thescanline is detected through absolute value detection of the form##EQU2## The Doppler values y are compressed and scaled using the CORDICprocessor of the detection and compression circuitry 428.

Once the Doppler signal amplitude values have been detected and filteredby FIR filter 432, the resulting values are spatially stored and imageclutter is removed by a flash suppression processor 434, whicheliminates large frame to frame variations in the displayed signals.Flash suppression processor 434 may operate by any of a number of knownflash suppression techniques, such as frame to frame comparison andelimination or the notch filtering technique of U.S. Pat. No. 5,197,477.A preferred technique for flash suppression processing is min-maxfiltering as described in detail in the parent U.S. patent applicationSer. No. 08/672,782.

The image frame memory 430 is capable of storing either a gray scaleframe or a power Doppler frame. Each frame can be temporally filtered bythe IIR filter 436, which performs frame averaging on a point-by-pointbasis as described above. The temporally filtered image information isthen provided to the back end ASIC 50 for scan conversion and display.

The sequences of operating the digital signal processing ASIC 40 for Bmode (two dimensional) echo and Doppler processing, respectively, areoutlined in the flowcharts of FIGS. 6 and 7, respectively. The number ineach flowchart block of FIGS. 6 and 7 refers to the numbered processorin the ASIC block diagram of FIG. 5.

The image frame memory 430 of the digital signal processing ASIC 40shares a common architecture and implementation technology with theframe buffer memory of the back end ASIC 50. To take advantage of thiscommonality and the resultant efficiency in ASIC fabrication anddensity, the image frame memory 430 and its associated flash suppressionprocessor 434 and IIR filter 436 can be located on the back end ASIC 50,thereby partitioning the digital signal processing ASIC and the back endASIC at the output of FIR filter 432. Thus, the digital signalprocessing function of FIG. 5 up through the output of FIR filter 432,or all of the functions shown in FIG. 5 can be fabricated on a singleintegrated circuit chip, depending upon this partitioning choice andother integrated circuit layout considerations.

The back end ASIC 50 is the location of the RISC processor 502, which isused to coordinate the timing of all of the operations of the handheldultrasound system. The RISC processor is connected to all other majorfunctional areas of the ASICs to coordinate process timing and to loadbuffers and registers with the data necessary to perform the type ofprocessing and display desired by the user. Program data for operationof the RISC processor is stored in a program memory 52 which is accessedby the RISC processor. Timing for the RISC processor is provided byclock signals from the clock generator located on the front end ASIC 30.The RISC processor also communicates through a PCMCIA and/or infraredtransmitter interface, by which the processor can access additionalprogram data or transmit image information remotely. The interface canconnect to a telemetry link or a modem for the transmission ofultrasound images from the handheld unit to a remote location, forinstance.

The RISC processor is operated under user control by commands andentries made by the user on the user control 70. A chart showing controlfunctions, the type of controls, and their description is shown in FIG.8. It will be appreciated that a number of functions, such as patientdata entry, Cineloop operation, and 3D review, will operate through menucontrol to minimize the number of key or button controls on the smallhandheld unit. To further simplify the unit a number of operatingfunctions are preprogrammed to specific diagnostic applications and willoperate automatically when a specific application is selected. Selectionof B mode imaging will automatically invoke frequency compounding anddepth dependent filtering on the digital signal processing ASIC 40, forinstance, while a four multiplier filter will automatically be set up asa wall filter on the DSP ASIC when Doppler operation is selected. Themenu selection of specific clinical applications can automaticallyinvoke specific feature settings such as TGC control characteristics andfocal zones, for example.

What is claimed is:
 1. For use in a handheld ultrasound device includinga transducer, beamformer and image processor, a digital signal processorcomprising:a digital B mode filter for filtering B mode signals; adigital Doppler filter for filtering Doppler signals; a B mode signaldetection and mapping circuit; and a Doppler signal estimator circuit.2. In the handheld ultrasound device of claim 1, wherein said digitalfilters, said B mode signal detection and mapping circuit, and saidDoppler signal estimator circuit are located on a digital signalprocessing integrated circuit chip.
 3. In the handheld ultrasound deviceof claim 1, wherein common elements of said digital signal processor areused for Doppler processing of Doppler signals, and also for B modeprocessing of B mode signals.
 4. In the handheld ultrasound device ofclaim 3, wherein said common elements include elements of said digitalfilter circuits.
 5. In the handheld ultrasound device of claim 4,wherein said digital signal processor includes a digital filter which isoperated as a bandpass filter for B mode signals, and as a high passfilter for Doppler signals.
 6. In the handheld ultrasound device ofclaim 4, wherein said digital signal processor includes a digital filterwhich is operated as a bandpass filter for B mode signals and as a wallfilter for Doppler signals.
 7. In the handheld ultrasound device ofclaim 1, wherein said digital signal processor further includes anormalization circuit for normalizing variations in the transduceraperture.
 8. In the handheld ultrasound device of claim 1, wherein saiddigital signal processor further includes means for forming a syntheticaperture.
 9. In the handheld ultrasound device of claim 1, wherein saiddigital signal processor further includes a depth dependent filter. 10.In the handheld ultrasound device of claim 1, wherein said digitalsignal processor further includes a speckle reduction circuit.
 11. Inthe handheld ultrasound device of claim 1, wherein said digital signalprocessor further includes a flash suppression circuit for Dopplersignals.
 12. In the handheld ultrasound device of claim 1, wherein saiddigital signal processor further includes means for assembling scanlinesfrom multiple focal zones.
 13. In a handheld ultrasound device asdefined by claim 1, the weight of the ultrasound device being less thanfive pounds.